This invention relates to voltage control systems of the tap changing type, and more particularly to an improved circuit for preventing arcing as taps are changed.
A step voltage regulator is an autotransformer provided with load ratio control equipment for regulating the voltage on the feeder or bus to which it is connected. A typical step voltage regulator may have a 100 percent exciting winding in shunt with the line on the source side and normally maintains the voltage on the load side within a desired voltage bandwidth by a 10 percent tapped buck/boost winding in series with the line. The series winding has taps connected to the stationary contacts of a tap changer dial switch having a pair of rotatable tap selectors driven by a reversible motor into sequential engagement with the taps and usually provides the ability to change the effective turns ratio from input to output plus or minus 10 percent in 32 steps of 5/8 percent voltage increments. The rotatable selectors are usually connected through collector rings to the opposite sides of a bridging center-tapped autotransformer reactor, termed a preventive autotransformer, to permit transition from one tap position to another without interrupting the load current. When the tap selectors bridge adjacent taps, the high reactance of the preventive autotransformer limits circulating current to a safe value and reduces burning and erosion of the tap changer contacts. It also provides a voltage midway between that of the physical taps to thereby provide twice the number of voltage steps. However, such a preventive autotransformer has continuous energy losses in operation and is bulky and expensive to construct, and U.S. Pat. No. 4,130,789, incorporated herein by reference, discloses a tap changing voltage regulator which eliminates such preventive autotransformer and also prevents arcing at the tap changer selector contacts by providing a half-tap voltage auxiliary winding and an auxiliary switch which permits a selector contact to step arclessly to an open circuited new tap, and then connects the selector contact in series with the auxiliary winding and a current-limiting inductor and the load at reduced voltage to effect a tap change without interruption of the load circuit. U.S. patent application Ser. No. 947,871, filed Oct. 2, 1978, and also incorporated herein by reference, teaches a voltage regulation system which allows for substantially arcless tap changing without utilizing a preventive autotransformer. While the inventions set forth in the foregoing patent and patent application provide greatly improved results over previously known voltage regulators, it has been found that some arcing still occurs under operating conditions.
It is therefore an object of this invention to provide an improved tap changer voltage regulator which eliminates arcing at the auxiliary contacts thereof and also arclessly interrupts the current which circulates when the selector contacts are on adjacent taps during a tap change.
Another object of the invention is to provide an economical and simplified circuit for an arcless tap changer which provides an acceptably small step change in voltage.